Additionally, at a polymeric solids content of about 55 per cent, latices of polymers containing functional groups tend to have a relatively high viscosity. This presents a drawback in the application of such latices in coating compositions as the viscosity of the composition should usually be low.
U.S. Pat. No. 3,637,563 issued Jan. 25, 1972, to Christena, assigned to the Celanese Corporation discloses a process for the manufacture of high solids acrylate latices. In the process, a pre-emulsion is made. The pre-emulsion is a water in hydrocarbon emulsion. The present invention does not require the use of such pre-emulsion.
British Pat. No. 1,191,649 published May 13, 1970, in the name of the Celanese Corporation discloses a process wherein a high solids vinyl acetate latex is produced by polymerizing a monomer emulsion in water in the presence of a minimum amount of surfactant and catalyst to form a relatively large particle size latex. At about 50 to 90 percent conversion, additional surfactant is added to further stabilize the latex originally formed. The present process contemplates the addition of the surfactant to create a new population of particles.
U.S. Pat. No. 4,130,523 issued Dec. 19, 1978, to Hoy et al, assigned to Union Carbide teaches a process for producing a high solids latex which requires the continuous withdrawal of a portion of the partially polymerized emulsion and adding fresh monomer dispersion. In the final step the withdrawn emulsion is returned to the reaction vessel. The present patent application does not require the withdrawl or cycling of partially polymerized monomer dispersion.
In theory, the method for increasing the solids content of a latex is to improve the degree of packing of the polymer particles. If the latex contains only particles of a relatively large uniform size, there are significant voids between the polymer particles which can now be filled with smaller particles.
The object of the prior art patents, and the present patent is to produce a high solids latex having a low viscosity. To do this, one must create a bimodal or polydisperse particle size distribution in the latex during polymerization. In the prior art, when latices of different particle sizes were blended and concentrated, solids levels could be increased when the diameters of the large particles was about 1600 A and the diameters of the small particles was about 900 A. The weight ratio of large to small particles was in the range of about 70:30 to 75:25.